In the case of MRI systems with relatively high main field strengths, (e.g., B0≥3 Tesla), the undesired inhomogeneity of the B1-RF field is a known problem that is caused by the interaction between the human tissue and the RF transmission fields of the transmission antennas, as a result of which a spatial variation in the field distribution is generated. This also includes phenomena such as, e.g., “dielectric resonance,” which may lead to field increases in the examination object.
The spatial variation of the RF fields leads to different flip angles during the excitation in MRI imaging sequences, and hence to a signal and contrast variation in the recorded images.
Different field distributions regularly emerge in patients, even in the case of nominally the same excitations (e.g., the CP mode explained below). In addition to the interaction of the patient with the fields, the interaction or reaction of the patient with the antenna, which becomes noticeable as a result of modified input impedances and couplings of the antennas, is an important cause in this respect. By way of example, if the input impedances of the antenna under load with the patient are different, a signal with the same amplitude, applied from the outside, will lead to fields with different amplitudes.
As a result, there may also be effects on the distribution of the specific absorption rate (SAR) of electromagnetic energy in tissue in addition to the field distortions if the excitation no longer corresponds to the desired excitation, e.g., the complex RF voltages of the RF excitation pulse sequences emitted at the transmission antenna(s).
The degree of the reaction of the patient on the antenna depends on the patient size, weight, tissue composition (e.g., proportion of muscle or fat), and the patient position relative to the antenna.
By way of example, the standard excitation is the circularly polarized excitation (CP mode), which, in a 2-channel system, is defined by virtue of the amplitudes of the two transmission channels having the same magnitude and a 90° phase difference. On the transmission antenna, the same currents set in on the rods along the circumference. In reality, variations in the rod currents, which exhibit a factor of 2 between the maximum and minimum current, are observed as a result of the reactions on the input impedances. As a result, the setting-in field distribution differs from the desired CP mode.
A solution to this problem lies in the use of multichannel transmission systems. Here, a plurality of transmission channels, (e.g., at least two), with different field distributions are available. These transmission channels may then be superposed in such a way that the field inhomogeneity is reduced. To this end, so-called “pre-scan” measurements are required prior to the actual examination of the patient, by which the field distribution in the presence of the patient is initially measured as a so-called “B1 map” (e.g., B1 RF field distribution map). Actuation parameters are determined for the transmission channels using the field distributions obtained thus, as a result of which, for example, the field homogeneity is improved. Two channels are not always sufficient for a real compensation of the field inhomogeneities, however.
DE 10254660 B4 discloses a method and a device for temporal correction of the field strength of radiofrequency pulses, which are emitted during a magnetic resonance measurement by a circularly polarizing birdcage antenna of a magnetic resonance measurement apparatus. The current flowing in the antenna during the emission of the radiofrequency pulses is regulated to a predetermined intended value with the aid of a reaction signal by varying power fed into the antenna. In order to form the reaction signal, output signals of two field probes disposed in the vicinity of the antenna at an angle with respect to one another or, alternatively, output signals of two directional couplers on the feed lines of the antenna are superposed with a fitting phase shift. A disadvantage here is that a very complicated closed-loop control circuit is required. This continuously regulates the antenna current, and hence the field strength, to a constant value during the examination of the object or the patient, leading to high development, production, maintenance, repair, and operating costs, and increases the examination time and the load on a patient.